Title: Stochastic FDTD Modeling of Propagation Loss due to Random Surface Roughness in Sidewalls of Optical Interconnects
The dielectric waveguide (WG) is an important building block of high-speed and high-bandwidth optical and opto-electronic interconnect networks that operate in the THz frequency regime. At the interface of Si/SiO 2 dielectric waveguides with width above w = 2.5 μm and anisotropic surface roughness, transverse electric (TE) mode surface wave propagation can experience a loss of approximately a = 2 dB/cm; however, propagation losses increase rapidly to near a = 44 dB/cm as the width decreases to w = 500 nm, due to increased interaction of surface waves and sidewall surface roughness that exhibits random distribution inherent to the manufacturing process. Previous works have developed analytic expressions for computing propagation loss in a single dielectric waveguide exhibiting random roughness. More recent works report a = 0.4 dB/cm noting the non-trivial estimation errors in previous theoretical formulations which relied on planar approximations, and highlight the discrepancy in planar approximations vs. the 3-D Volume Current Method. A challenge that remains in the path of designing nanoscale optical interconnects is the dearth of efficient 3-D stochastic computational electromagnetic (CEM) models for multiple tightly coupled optical dielectric waveguides that characterize propagation loss due to random surface roughness in waveguide sidewalls. Through a series of theoretical and numerical experiments developed in the method of finite-difference time-domain (FDTD), we aim to develop stochastic CEM models to quantify propagation loss and facilitate signal & power integrity modeling & simulation of arbitrary configurations of multiple tightly-coupled waveguides, and to gain further insights into loss mechanisms due to random surface roughness in optical interconnects. more »« less
Electromagnetic (EM) scattering may be a significant source of degradation in signal and power integrity of high-contrast silicon-on-insulator (SOI) nano-scale interconnects, such as opto-electronic or optical interconnects operating at 100 s of THz where two-dimensional (2D) analytical models of dielectric slab waveguides are often used to approximate scattering loss. In this work, a formulation is presented to relate the scattering (propagation) loss to the scattering parameters (S-parameters) for the smooth waveguide; the results are correlated with results from the finite-difference time-domain (FDTD) method in 2D space. We propose a normalization factor to the previous 2D analytical formulation for the stochastic scattering loss based on physical parameters of waveguides exhibiting random surface roughness under the exponential autocorrelation function (ACF), and validate the results by comparing against numerical experiments via the 2D FDTD method, through simulation of hundreds of rough waveguides; additionally, results are compared to other 2D analytical and previous 3D experimental results. The FDTD environment is described and validated by comparing results of the smooth waveguide against analytical solutions for wave impedance, propagation constant, and S-parameters. Results show that the FDTD model is in agreement with the analytical solution for the smooth waveguide and is a reasonable approximation of the stochastic scattering loss for the rough waveguide.
Correas-Serrano, Diego; Paul, Nayan; Gomez-Diaz, J. Sebastian(
, Micro- and Nanotechnology Sensors, Systems, and Applications XI, edited by Thomas George, M. Saif Islam, Proc. of SPIE)
We explore the possibilities enabled by the spatiotemporal modulation of graphene’s conductivity to realize magnetic-free isolators at terahertz and infrared frequencies. To this purpose, graphene is loaded with periodically distributed gates that are time-modulated. First, we investigate plasmonic isolators based on various mechanisms such as symmetric bandgaps and interband photonic transitions and we demonstrate isolation levels over 30 dB using realistic biasing schemes. To lessen the dependence on high-quality graphene able to support surface plasmons with low damping, we then introduce a hybrid photonic platform based on spatiotemporally modulated graphene coupled to high-Q modes
propagating on dielectric waveguides. We exploit transversal Fabry-Perot resonances appearing due to the finite-width of the waveguide to significantly boost graphene/waveguide interactions and to achieve isolation levels over 50 dB in compact structures modulated with low biasing voltages. The resulting platform is CMOS-compatible, exhibits an overall loss below 4 dB, and is robust against graphene imperfections. We also put forward a theoretical framework based on coupled-mode theory and on solving the eigenstates of the modulated structure that is in excellent agreement with full-wave numerical simulations, sheds light in the underlying physics that govern the proposed isolators, and speeds-up their analysis and design. We envision that the proposed technology will open new and efficient routes to realize integrated and silicon compatible isolators, with wide range of applications in communications and photonic networks.
Reddy, Jay W.; Nelson, Sarah; Chamanzar, Maysamreza(
, Optics Express)
We demonstrate novel trapezoidal and rectangular stratified trench optical waveguide designs that feature low-loss two-dimensional confinement of guided optical modes that can be realized in continuous polymer thin film layers formed in a trench mold. The design is based on geometrical bends in a thin film core to enable two-dimensional confinement of light in the transverse plane, without any variation in the core thickness. Incidentally, the waveguide design would completely obviate the need for etching the waveguide core, avoiding the scattering loss due to the etched sidewall roughness. This new design exhibits an intrinsic leakage loss due to coupling of light out of the trench, which can be minimized by choosing an appropriate waveguide geometry. Finite-difference eigenmode simulation demonstrates a low intrinsic leakage loss of less than 0.15 dB/cm. We discuss the principle of operation of these stratified trench waveguides and present the design and numerical simulations of a specific realization of this waveguide geometry. The design considerations and tradeoffs in propagation loss and confinement compared with traditional ridge waveguides are discussed.
Tan, Si; Deng, Huiyang; Urbanek, Karel E.; Miao, Yu; Zhao, Zhexin; Harris, James S.; Byer, Robert L.(
, Optics Express)
The unique properties of gallium oxide (GaOx) have drawn increasing interest as a material suitable for high-power electronic and optical applications. Herein, we report the demonstration of low-loss GaOx-core/SiO2-cladding waveguides on Si substrate. We present the fabrication process and annealing treatments of the waveguide devices, and we characterize the corresponding effects on optical transmission for 3 common wavelengths: 633 nm, 1064 nm, and 1550 nm. The best propagation loss achieved for these wavelengths is measured to be,, and, respectively. We discuss the major waveguide loss mechanisms, followed by results of pump and probe experiments using visible/IR wavelengths for waveguides treated under various post-fabrication annealing conditions. We also show nonlinear measurements for a 250 fs laser beam to offer additional insights into the loss mechanisms, which are consistent with the linear optical transmission performances. High waveguide laser-induced damage threshold (LIDT) ofis measured at this pulse width, making GaOxa potential candidate for high-power integrated photonic devices.
We report an advanced Fourier transform spectrometer (FTS) on silicon with significant improvement compared with our previous demonstration in [Nat. Commun.9,665(2018)2041-1723]. We retrieve a broadband spectrum (7 THz around 193 THz) with 0.11 THz or sub nm resolution, more than 3 times higher than previously demonstrated [Nat. Commun.9,665(2018)2041-1723]. Moreover, it effectively solves the issue of fabrication variation in waveguide width, which is a common issue in silicon photonics. The structure is a balanced Mach–Zehnder interferometer with 10 cm long serpentine waveguides. Quasi-continuous optical path difference between the two arms is induced by changing the effective index of one arm using an integrated heater. The serpentine arms utilize wide multi-mode waveguides at the straight sections to reduce propagation loss and narrow single-mode waveguides at the bending sections to keep the footprint compact and avoid modal crosstalk. The reduction of propagation loss leads to higher spectral efficiency, larger dynamic range, and better signal-to-noise ratio. Also, for the first time to our knowledge, we perform a thorough systematic analysis on how the fabrication variation on the waveguide widths can affect its performance. Additionally, we demonstrate that using wide waveguides efficiently leads to a fabrication-tolerant device. This work could further pave the way towards a mature silicon-based FTS operating with both broad bandwidth (over 60 nm) and high resolution suitable for integration with various mobile platforms.
Guiana, Brian, and Zadehgol, Ata. Stochastic FDTD Modeling of Propagation Loss due to Random Surface Roughness in Sidewalls of Optical Interconnects. Retrieved from https://par.nsf.gov/biblio/10282506. 2021 United States National Committee of URSI National Radio Science Meeting (USNC-URSI NRSM) . Web. doi:10.23919/USNC-URSINRSM51531.2021.9336433.
Guiana, Brian, & Zadehgol, Ata. Stochastic FDTD Modeling of Propagation Loss due to Random Surface Roughness in Sidewalls of Optical Interconnects. 2021 United States National Committee of URSI National Radio Science Meeting (USNC-URSI NRSM), (). Retrieved from https://par.nsf.gov/biblio/10282506. https://doi.org/10.23919/USNC-URSINRSM51531.2021.9336433
Guiana, Brian, and Zadehgol, Ata.
"Stochastic FDTD Modeling of Propagation Loss due to Random Surface Roughness in Sidewalls of Optical Interconnects". 2021 United States National Committee of URSI National Radio Science Meeting (USNC-URSI NRSM) (). Country unknown/Code not available. https://doi.org/10.23919/USNC-URSINRSM51531.2021.9336433.https://par.nsf.gov/biblio/10282506.
@article{osti_10282506,
place = {Country unknown/Code not available},
title = {Stochastic FDTD Modeling of Propagation Loss due to Random Surface Roughness in Sidewalls of Optical Interconnects},
url = {https://par.nsf.gov/biblio/10282506},
DOI = {10.23919/USNC-URSINRSM51531.2021.9336433},
abstractNote = {The dielectric waveguide (WG) is an important building block of high-speed and high-bandwidth optical and opto-electronic interconnect networks that operate in the THz frequency regime. At the interface of Si/SiO 2 dielectric waveguides with width above w = 2.5 μm and anisotropic surface roughness, transverse electric (TE) mode surface wave propagation can experience a loss of approximately a = 2 dB/cm; however, propagation losses increase rapidly to near a = 44 dB/cm as the width decreases to w = 500 nm, due to increased interaction of surface waves and sidewall surface roughness that exhibits random distribution inherent to the manufacturing process. Previous works have developed analytic expressions for computing propagation loss in a single dielectric waveguide exhibiting random roughness. More recent works report a = 0.4 dB/cm noting the non-trivial estimation errors in previous theoretical formulations which relied on planar approximations, and highlight the discrepancy in planar approximations vs. the 3-D Volume Current Method. A challenge that remains in the path of designing nanoscale optical interconnects is the dearth of efficient 3-D stochastic computational electromagnetic (CEM) models for multiple tightly coupled optical dielectric waveguides that characterize propagation loss due to random surface roughness in waveguide sidewalls. Through a series of theoretical and numerical experiments developed in the method of finite-difference time-domain (FDTD), we aim to develop stochastic CEM models to quantify propagation loss and facilitate signal & power integrity modeling & simulation of arbitrary configurations of multiple tightly-coupled waveguides, and to gain further insights into loss mechanisms due to random surface roughness in optical interconnects.},
journal = {2021 United States National Committee of URSI National Radio Science Meeting (USNC-URSI NRSM)},
author = {Guiana, Brian and Zadehgol, Ata},
editor = {null}
}
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